There are many applications where it is desirable to determine various characteristics of a fluid, such as the concentration of material in a fluid solution or the density of a fluid solution, where the measurement technique is noninvasive and does not expose workers to toxic fumes or spills, radioactive contamination and the like. There are existing methods that use sonic and ultrasonic excitation to measure sound velocity and concomitant solution density and/or solute concentration. The known techniques rely on the formation of standing waves in a liquid and are generally referred to as acoustic or ultrasonic interferometers. As noted by A. L. Loomis et al., "A Sonic Interferometer for Measuring Compressional Velocities in Liquids: A Precise Method," 17 J. Opt. Soc. Am., pp. 295-307 (1928), ultrasonic frequencies are preferred where the wavelength in the fluid is small compared with the diameter of the liquid column so that the sound velocity in the fluid is highly independent of the dimensions and the material of the vessel containing the fluid.
Typically, the instruments that measure the sound velocity use special sampling cells adapted to a particular technique and require that the excitation devices be suspended in the fluid. One particular device, described in G. L. Gooberman, Ultrasonics Theory and Applications, pp. 146-150, The English Universities Press, Ltd., London, United Kingdom (1968), provided a fixed-position transducer and a variable reflector in a liquid medium. The frequency of the transducer output was varied and its impedance was measured. The impedance measurement underwent minima and maxima, i.e., resonances, as the frequency was varied because of the force established by the standing wave pattern. Thus, frequencies for maximum impedance were measured and the frequency difference between adjacent maxima was related to the sound velocity in the liquid. The sound velocity was then related to fluid density and/or solute concentration in the fluid.
For a production method and apparatus, it would be desirable to provide a noninvasive technique that uses the fluid container walls as the interferometer. Accordingly, it is an object of the present invention to provide transducers external to a fluid container containing the fluid to be measured.
It is another object of the present invention to provide for measuring fluid characteristics without exposing workers to the fluid.
Yet another object of the present invention is to provide a measurement instrument that can be readily adapted to process pipes and storage vessels.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.